1. Field of the Invention
The present invention relates generally to the field of optical communications systems and, more particularly, the present invention relates to a method for readjusting the phase or frequency modulation shift of an optical transmission signal.
2. Description of the Related Art
Various modulation schemes for optical communications systems are known in the art. Frequency or phase modulation are utilized in optical communications technology in addition to intensity or amplitude modulation. One type of frequency modulation is binary frequency shift keying or (FSK). In FSK or CPFSK, (Continuous Phase Frequency Shift Keying) the optical signal is represented by the time-dependent equation for field strength normed to an amplitude value of 1: ##EQU1## Where .omega..sub.0 is the optical frequency of the optical oscillation and .DELTA..omega. is the frequency shift which can be ideally achieved between the two symbols, also known as the rated or desired shift value. .epsilon. is zero when the ideal shift or rated shift is set and is not equal to zero when the frequency shift deviates from the rated shift value by .DELTA..omega. whereby the deviation from the rated shift value is established by .epsilon..multidot..DELTA..omega. and the equation .vertline..epsilon..vertline.&lt;&lt;f1 usually applies.
Normally, the signal is in Non-Return-to-Zero (NRZ) format wherein d(t)=0 is valid for a binary "0" and d(t)=1 is valid for a transmitted binary symbol "1" during the entire duration of a bit T (see for example, R. S. Vodhanel, Electronics Letters, Vol. 24 No. 3, pp. 163-165 (1988). In some situations, a binary "1" is represented by alternate positive and negative frequency shifts (see R. Noe et al. Proc. ECOC Vol. 1 pages 175-188 (1988) Brighton). This is referred to as Alternate Mark Inversion or AMI. In an AMI-FSK signal, d(t)=0 is valid for a transmitted binary "0" and d(t)=1 or d(t)=-1 is valid alternately for a transmitted binary "1" during the entire duration of a bit T. Some applications require bipolar FSK rather than AMI-FSK, however, because the signal statistics of AMI-FSK and bipolar FSK are identical, a control modulation shift which is designed for AMI-FSK is also suitable for bipolar FSK.
Another modulation scheme for use in optical communications is binary Phase Shift Keying PSK (see for example, E, Gottwald et al. Proc. ECOC Amsterdam at 331-34 1990). Binary Differential Phase Shift Keying, or DPSK is also used in such systems. (see E. Meisner, H. Rodler, Proc. EFOC/LAN Paris paper 123 at 378-81 1992; T. Naito et al. Electronics Letters, Vol. 26 No. 20 at 1734-1736 1990; E. Meisner et al. Proc. ECOC, paper We A8.2, Berlin 1992). Phase modulation for these systems may be accomplished by either modulation of the phase in an external modulator (see i.e. E. Gottwald et al. Proc. ECOC Amsterdam at 331-334 (1990); E. Meisner, H. Rodler Proc. EFOC/LAN Paris paper 123, at 378-81 (1992) or directly by modulation of the transmission light with a signal that corresponds to the first chronological derivation of the desired phase (see T. Naito et al. Electronics Letters, Vol. 26 No. 20 at 1734-1736 1990; E. Meisner et al. Proc. ECOC, paper We A8.2, Berlin 1992). This relationship is known because the phase is the integral of the radian frequency of the transmission light. In the second form of presentation, the transmission light is represented by Equation 1 above. Equation (2), .DELTA..omega..multidot.T=.pi. is selected so that the phase shift or rated shift value is achieved within T of a bit based on frequency shift amounts to .pi. insofar as the pulse integral d(t) is the same over the duration T of the bit.
When each phase shift is in the same direction, it is considered PSK or DPSK. If a phase modulator is to be used, it must have an infinite range because an arbitrary range of binary symbols "1" may occur over time. Generally, this phase shift is accomplished by applying a short current pulse to the semiconductor laser that generates the transmission light. The frequency is briefly modified as a result of the phase shift and returns to its original value after the end of the pulse which varies the phase. (see T. Naito et al. Electronics Letters, Vol. 26 No. 20 at 1734-1736 1990). For a transmitted binary symbol without phase shift, d(t)=0 is valid and for a transmitted binary symbol having a phase shift, d(t) is a pulse having the area T that is preferably short in order to achieve rapid phase shifting.
In many applications the phase shifts have alternate positive and negative polarity. For example, this is true where a phase modulator is driven with an NRZ signal (see. E. Gottwald et al. Proc. ECOC Amsterdam at 331-334 (1990); T. Naito et al. Electronics Letters, Vol. 26 No. 20 at 1734-1736 1990). Direct modulation of a laser on the basis of alternating positive and negative pulses is also possible (see E. Meisner et al. Proc. ECOC paper We A8.2 Berlin 1992). For a transmitted binary symbol without phase shift, d(t)=0 applies for a transmitted binary symbol with phase shift, d(t) is a pulse that comprises the area T or alternately -T, this being optimally short in order to achieve faster phase shifting.
In known frequency and phase modulation systems for optical communication systems, the phase modulation or frequency modulation shift varies as a result of a variety of influences so that the communication transmission quality of an optical waveguide link deteriorates over time. The present invention provides a solution to this problem. Although binary modulation schemes are described for the purpose of practicing the invention, it will be understood that the invention is also suitable for multi-stage digital, analog frequency or phase modulation systems as well.